A plane radio wave in air has a peak electric field of 15 V/m and a wavelength of 2.5 m…

Step 1. The magnetic field. We want $B_0$, so $B_0 = \dfrac{E_0}{c} = \dfrac{15\ \text{V/m}}{3.00 \times 10^{8}\ \text{m/s}} = 50 \times 10^{-9}\ \text{T} = 50\ \text{nT}$.

Step 2. The frequency. $f = \dfrac{c}{\lambda} = \dfrac{3.00 \times 10^{8}\ \text{m/s}}{2.5\ \text{m}} = 120 \times 10^{6}\ \text{Hz} = 120\ \text{MHz}$.